FIG. 1A shows a cantilever beam accelerometer 100, which includes a structure layer 101, a sensing unit 102, and an anchor 103. The structure layer 101 is rectangular in form and attached on one end to the anchor 3. The sensing unit 102, which is attached to the structure layer 101, senses a movement of the structure layer 101, which is free to move at the end opposite the anchor 103. Here, the sensing unit 102 may operate, for example, in a piezoelectric or piezoresistive manner.
FIG. 1B shows the cantilever beam accelerometer 100 of FIG. 1A with a proof mass 104 attached to its tip to generate more strain along the structure layer 100, which by its attachment to the anchor 103, forms a suspended beam to support the proof mass 104. As a result, the addition of the proof mass 104 may provide a more sensitive operation of the accelerometer of FIG. 1B as compared to the accelerometer of FIG. 1A.
Although the design of the cantilever beam accelerometers of FIGS. 1A and B may be relatively simple and easy to fabricate, such designs may not be suitable for sensing acceleration in three axial directions. In particular, such designs may not provide a differential output signal in order to minimize noise.
FIG. 2 shows an accelerometer 200 based on a sensing membrane and a center proof mass, which is discussed, for example, by Li-Ping et al., J. MEMS, 2003, Volume 12, pages 433 to 439. The accelerometer 200 includes a sensing membrane, a center proof mass, a ring-shaped top electrode, a piezoelectric layer, and a mounting frame. Here, the balanced structure of accelerometer 200 minimizes cross sensitivity, and the extra mass significantly improves the overall sensitivity as compared to the designs shown in FIGS. 1A and 1B.
FIGS. 3A-C show top and side views of a tri-axis accelerometer 300 based on a piezoresistive sensing unit on top of a membrane, which is referred to, for example, by U.S. Patent Application Publication No. 2004/0027033. The tri-axis accelerometer 300 includes a disk-shaped suspension membrane 301, a proof mass 302 attached underneath the disk-shaped membrane 301, and piezoresistive sensing units 303 arranged on the surface of the disk-shaped membrane 301.
The designs shown in FIGS. 2 and 3A-C may be used for tri-axis sensing, but the suspension parts used in these designs are configured as whole membranes and therefore are relatively stiff. Here, the sensitivity of the piezoelectric sensing or piezoresistive sensing depends on the strain generated by the external acceleration. Therefore, a relatively stiff structure may not provide enough sensitivity. Moreover, having a strip or disc-like design may lead to cross talk since an acceleration in one direction, for example, may result in a deformation of the sensing films in other directions thereby requiring a more complicated signal processing to achieve a clean signal. Moreover still, devices having a suspension membrane design may occupy significant space on a microchip, which may increase the cost of such devices.